Infantile nephropathic cystinosis is a fatal inborn error of metabolism for which the basic biochemical defect is unknown. Cells derived from cystinotic patients accumulate cystine within lysosomes to 100 times the amount found in lysosomes of normal tissue. We have shown that treatment of cystinotic fibroblasts with cysteamine produces rapid cystine depletion. Study of cystine depleted cystinotic fibroblasts has shown that cystine reaccumulates from the degradation of cellular proteins. I have recently found that addition of bovine serum albumin (BSA) to cystine depleted cystinotic fibroblasts incubated in cystine-free medium results in an increased rate of cystine accumulation proportional to the concentration of BSA. This supplementation permits prolonged suvival of both normal and cystinotic fibroblasts in cystine free medium. Study of the rate of degradation of cellular proteins and of the rate of pinocytosis by normal and cystinotic fibroblasts did not demonstrate significant differences. However, degradation of 125I-BSA was twice as rapid in cystinotic cells as in normal cells. Prereduction of BSA with dithiothreitol markedly reduced the amount of cystine accumulation produced. I propose to study the uptake and degradation of 125I-labelled disulfide and sulfhydryl containing proteins by both normal and cystinotic fibroblasts to determine if the cystine accumulation is related to: 1) an excessive rate of proteolysis of extracellular protein by the cystinotic cells, 2) a defective lysosomal disulfide reductase, and 3) a defective lysosomal transport for cystine and/or cysteine. Information will also be gained on the effect the disulfide content of a protein has on its rate of uptake and degradation.